Japanese patent application publication Number JP2006-511078A1 discloses a supply assembly (hereinafter referred to as “lighting device”) for an LED (a light emitting diode) lighting module for supplying an electric power.
This lighting device includes, as shown in FIG. 10, a series circuit of, a diode D100 and a control switch 101 of a MOSFET, connected between both ends of a DC power supply 100. A series circuit of an inductor L100 and a LED lighting module 102 is connected between both ends of the diode D100.
A controller 103 is adapted to generate a dual PWM switching signal and supply the signal to an input of the control switch 101 through an amplifier 104. The dual PWM switching signal is composed of a low frequency busts including high frequency pulses.
The controller 103 includes a current mode pulse width modulator 105. The current mode pulse width modulator 105 is configured to receive: an LED current reference signal from a power source 106 a detected current; and a high frequency saw-teeth wave. The current mode pulse width modulator 105 applies a high frequency pulse width modulation switching signal to one of inputs of an AND gate 107, and output signal of the AND gate 107 is supplied to a gate of the control switch 101 through the amplifier 104.
Therefore, this lighting device can change an average current through the LED lighting module 102, in order to vary the light intensity outputted by the LED lighting module 102, by varying the low frequency component of the dual PWM switching signal.
In what is called a burst dimming system for dimming such that a load current intermittently flows through the LED lighting module 102, it is necessary to set a burst frequency to a comparatively low frequency (about 120 Hz) in order to dim the luminous flux low level.
In the burst dimming system, because the load current intermittently flows through the LED lighting module, there is an idle period during which no load current flows through the module. Therefore, there is a concern of visible flicker caused by interference with a specific frequency of video equipment (e.g. a video camera) when the LED lighting module is seen through the video equipment.
By the way, in an amplitude control system for varying a peak value of a load current through an LED lighting module, a load current continuously flows through the LED lighting module. Therefore, in the amplitude control system, it is possible to reduce a possibility of generation of visible flicker when the LED lighting module is seen through the video equipment.
However, if the amplitude control system is applied to an LED lighting module having an LED array, there is a problem that elements (LEDs) of the LED array each have dispersion in brightness. Because the elements of the LED array have dispersion in a forward voltage, and may have 15% or more dispersion in some array or element types, and therefore a voltage applied to the LED lighting module may decrease when luminous flux from the module under dimming is low. The problem becomes more marked as the number of LEDs connected in series is increased.